I hope it is a simple question...what mosfet would i use to switch a voltage between 3-12 volt? And how would the
Circuit look when it should drive an LED?? The gate would get be 5v from something like an arduino pin

I have some 10mm LEDs that are 3.3V and about 100mA. So, the mosfet will only have to switch
that, but maybe two of them. There will be several mosfets each driving one or two LEDs, that is why
I need an external 3.3V power supply and not just use the 5V from the controller.

I have some 10mm LEDs that are 3.3V and about 100mA. So, the mosfet will only have to switch
that, but maybe two of them. There will be several mosfets each driving one or two LEDs, that is why
I need an external 3.3V power supply and not just use the 5V from the controller.

All components will be through hole

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For through hole, you can use 2N7000; they're the least expensive N channel MOSFET I've found, about a nickel each in qty 100.

You can drive multiple MOSFETs from your controller, depending on switching frequency.

It sounds as if you want to drive the LEDs directly from a 3.3V supply; it's more typical to use a current limiting resistor to limit current.

Okay, so it is normal to use 5V with a resistor to get 3.3V? Can I ask why not just use 3.3V from the get go?

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The 3.3V figure you're using is probably the typical forward voltage. Some LEDs will be higher and some will be lower. To get a specific LED current (which would give more closely matched brightness), you either use a current source or a current limiting resistor.

In extremely cost sensitive applications, the resistors are sometimes omitted. Though that's not a good engineering practice.

Also, could you show me a schematic? Would I need pull-down resistors etc?

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We usually prefer to help people, not do the work for them. Here's something I had drawn for someone else; it shows the parasitic body diode explicitly: